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Abstract: Evolution of Anoxic Conditions in a Backbulge Basin, An Example: Pilot Basin, Utah and Nevada

WILLIAMSON, CHARLES B., University of Utah, Department of Geology and Geophysics, Salt Lake City, UT

Anoxic black shale deposits of the Paleozoic in western North America are often found in close temporal and spatial proximity to open ocean carbonate bank sedimentation. Although this type of deposition is common during much of the Paleozoic and Mesozoic stratigraphic record, it is not observed in modern oceanic sedimentation. Modern oceanic carbonate bank deposits are restricted to western margins of ocean basins, while in the eastern portions, high nutrient content waters preclude the formation of bank carbonates. Potential causes for black shale carbonate associations include several processes. Rapid sea level change could place shales in close proximity to carbonate banks in relatively short periods of time. Global climatic change, such as alteration of trade wind intensity can influence upwelling and allow for rapid changes in bottom water oxygen concentrations. Climatic change could also induce changes in water circulation patterns within basins which could have drastic effects in depositional patterns. The objectives of this project are to stratigraphically document bottom water oxygenation conditions occurring during deposition of the Pilot Shale using, iron, sulfate, and trace metal systematics. Nitrogen isotope analysis, used as a proxy for paleoproductivity in studies of other North American Devonian basins such as the Williston and Exshaw Basins, will be applied to the Pilot Shale. Results of geochemical and stratigraphic analyses along with paleogeographic reconstruction of the Pilot Basin will be compared in order to ascertain whether the cause of black shale deposition is a local phenomenon or related to regional oceanic conditions.

AAPG Search and Discovery Article #90931©1998 AAPG Foundation Grants-in-Aid